To be fair, a lot of people miss that Bootstrap is themeable, and what you are complaining about is just the default theme. E.g. check out https://bootswatch.com for examples, many aren't flatsville.

I've had confused conversations with UX, etc over the years when they say "We can't use Bootstrap - our colors are totally different!", etc.

I'm hearing I need to downgrade the Bootstrap in my suite? (A young addition).

Which is great because Bootstrap 2-3, whenever I used it, was pleasant.

I wasn't aware of the development timeline, thanks for the lead. Not flat is appealing.

I picked ExtJS 2.0.2 for a specific reason, seemingly rando old trusty version as base.

https://bootswatch.com/spacelab/ is the closest i've found to the original 2.23 theme which was the perfect blend of 3D and colours to my eyes.

bootstrap 5.3 optionally supports gradients natively and uses shadows on their homepage https://getbootstrap.com/

Agreed.

You don't need a css framework to do a flat UI, just remove all of the borders from every element.